A raster is a pattern of parallel lines comprising the display area of a display device. A raster display is effected by sequentially and selectively brightening or otherwise causing to be displayed points of the raster lines. The most well known raster display device is a cathode ray tube in which an electron beam is constrained to trace a raster by means of a high speed horizontal deflection circuit and a lower speed vertical deflection circuit. A selected display is caused by supplying a sequence of bright-up pulses to the brightness control of the beam as it traces the raster. Another prior art raster display device is a matrix of light emitting diodes arranged in rows and columns and addressed sequentially row by row and diode by diode within a row by means of a multiplexing arrangement.
The advantage of raster display devices is the simplicity of their control but this is achieved at the cost of some inflexibility in the display itself. This inflexibility is most apparent in interactive display terminals which are intended to permit a user to select and modify the display at will. Heretofore, interactive raster display devices have been restricted to the simplest of displays, those comprised of a font of alphanumeric characters which can be precoded and stored at the display terminal.
Recently it has been proposed to display and modify more complex images by using data processing apparatus to compute the form of the display line-by-line as the raster lines are being traced. One example of such apparatus is the raster vector generator disclosed in copending British patent applications 49780/74 and 20485/75 and in U.S. Pat. Nos. 3,906,480; 3,883,728; and 3,895,357; and in copending U.S. Pat. application Ser. No. 478,816 filed 6/11/74 and now abandoned, all of which are assigned to the instant assignee. In the raster vector generator and similar apparatus the computation of the display is a race against the steadily advancing raster trace. If a large number of computations have to be done for a line there is the probability that the raster scan will reach the line before the computations are complete.
One solution to the problem is to do only a certain number of computations per line and to display the result, the next image frame being reserved for the remaining computations and the resultant display. The image seen by the user is the superposition of the displays of succeeding frames. This solution complicates the control apparatus and could lead to image flicker.